The structure and function of semiconductor photodiodes are well known. Photodiodes convert photons into electrical energy. Conventional photodiodes operate in the visible and near-infrared range of the electromagnetic radiation spectrum. The particular semiconductor materials used determine the particular wavelength or wavelength range of the radiation to which the photodiode responds. Photodiodes can be fabricated from elemental semiconductors, such as silicon, as well as compound semiconductors, such as gallium-arsenide.
Photodiodes are typically either vertical P+N or N+P structures. Although a P+N diode is described in this paragraph, by reversing the diffusion types, an N+P diode will be formed. (This is also called a PIN structure, with “I” indicating the intrinsic layer). A conventional P+N photodiode includes a surface P-type anode region to which an anode contact is formed. An antireflective film generally overlies the P-type region to assure a high degree of transmission of radiation at the wavelength that the photodiode is designed to absorb. Beneath the P-type region is a very lightly-doped N-type drift region (also called the intrinsic region or space charge region in the literature) in which photons of the incident radiation are absorbed, generating hole-electron pairs. Adjoining the N-type drift region is a heavily-doped N+ cathode region, to which a cathode contact is formed at a surface of the device. In operation the P+N junction between the P-type anode region and the N-type drift region is reverse biased by an applied potential expanding the depletion layer on both sides of the junction. Because the N-type drift region is relatively lightly doped, the depletion layer is predominantly on the N-type side of the junction extending deeply into the drift region. Holes and elections generated in the depletion layer are swept in opposite directions in response to the applied potential, thus providing a current that is a function of the intensity of the incident radiation.
In many applications photodiodes are formed on integrated circuit die. As a result, when integrating photodiodes on the same semiconductor chip as other circuit elements such as transistors and resistors to perform complex functions in response in part to incident radiation signals, the constraints of the process for making such other elements must be considered in the design of the photodiode. It is desirable to minimize the complexity of a semiconductor fabrication process while maximizing the flexibility available to the designer to provide complex functionality in the device design. The inclusion of a photodiode on an integrated circuit chip made with state-of-the-art CMOS or BiCMOS process technology contributes to the foregoing design considerations.
Photodiodes in certain applications must be efficient, and/or provide substantially the same efficiency, over a relatively wide range of wavelengths, such as blue to red light, even in the face of changing process parameters. FIG. 1 shows the normalized optical response of an exemplary photodiode having a 3.sup.rd order anti-reflective coating (ARC) optimized for blue light, for red (650 nm), near infrared (IR; 780 nm) and blue (405 nm) light as a function of silicon dioxide thickness (in microns) above the photodiode. The oxide film stack over the photodiode comprising the inter-level dielectric (ILD) is a by-product of forming the multi-level metal interconnect. Because the ARC layer is generally optimized for blue light, the optical response at the three different wavelengths (such as used in optical storage products) can be seen to be a strong function of the oxide thickness over the photodiode, especially for red and IR.
Moreover, the thickness of respective oxide films forming the ILD are not well controlled due to thickness variability predominantly due to ILD layer deposition and planarization processing. Oxide (or other dielectric) thickness variability is generally both across a wafer or die, wafer to wafer, and lot-to-lot. Such variability in the thickness of layers disposed on top of the photodiode ARC can result in substantial loss and/or variation in efficiency. What is needed is a new photodiode architecture which provides multiple wavelength operation and a manufacturable process for forming the same.